1. Field of the Invention
The present invention relates to the field of battery load management in battery powered systems.
2. Prior Art
In various battery powered devices, there are a number of subsystems that need power. Typically at least one of the subsystems can operate at a reduced power if necessary. Such devices include devices with a camera flash, using a Xenon or LED Flash element, including cell phones, and TDMA transmission systems, where a capacitor is charged to provide power amplifier power during the TDMA transmission burst.
Taking devices with a camera flash, it is desirable to charge the flash capacitor as quickly as possible. However during high load currents, the system voltage will momentarily drop due to the internal ESR (equivalent series resistance) of the battery, together with any serial impedance between the battery and the load. For equipment requiring a minimum voltage for stable operation, the ESR of the system (battery plus serial impedance) needs to be calculated in order to estimate the maximum current that can be drawn from the battery without causing system voltage drop below the threshold for stable operation.
If the above is not done, then two scenarios are possible:
1. The system cut-off voltage, a pre-determined voltage at which the system ceases operation and powers down, will have to be set artificially high, to provide sufficient margin above the point at which the battery no longer reliably operates. The result is reduced operating time.
2. In a cell phone, high current peaks cause erratic behavior, or possibly even phone shut-off, even though the battery has plenty of remaining capacity.
For the foregoing reasons, it is desirable to limit the battery current drain of flash capacitor charging systems, when necessary, to below the nominal design value to maintain the required battery voltage for operation of other subsystems, thereby extending the useful battery life, or time between charges for rechargeable batteries.
In the prior art, additional hardware and software were added. The hardware added is a high precision high speed ADC. During the initial loading of the system, the voltage drop of the battery is measured with the ADC, and using software the battery ESR is calculated, from which the maximum load that the battery is able to support is determined.